The present invention relates to novolak type phenolic resins, and particularly to high-molecular weight novolak type phenolic resins which are very low in content of ionic impurities, high in proportion of methylene groups bonded at ortho--ortho position, and are soluble in solvents, and thus are excellent in handleability, curability, electric characteristics, and heat resistance characteristics and suitable in a wide variety of fields such as molding materials, epoxy resin curing agents, photoresist resins, base resins for carbon materials, thermoplastic resin modifying agents, rubber compounding, etc.
Conventionally known novolak type phenolic resins include those of random novolak type in which methylene groups bond at ortho-position and para-position at nearly the same ratio and those of high-ortho novolak type in which methylene groups bond predominantly at ortho-position. Random novolak type phenolic resins are usually obtained by subjecting a phenol compound and an aldehyde compound to addition condensation reaction using a known organic acid and/or an inorganic acid as a catalyst at 100.degree. C. for several hours under normal pressures, followed by dehydration and removal of unreacted monomers. High-ortho novolak type phenolic resins are usually obtained by subjecting a phenol compound and an aldehyde compound to addition condensation reaction using a metallic salt catalyst such as zinc acetate, lead acetate, zinc naphthenate or the like in weakly acidic state, allowing the dehydration condensation reaction to proceed directly or with further addition of an acid catalyst, and, if necessary, removing unreacted materials. (See, for example, JP-A-55-90523, JP-A-59-80418, and JP-A-62-230815.)
The random novolak type phenolic resins have the problems that they are low in speed of curing with e.g. hexamethylenetetramine, and the high-ortho novolak type phenolic resins obtained using metallic salt catalysts have the problems that metallic ions are contained in the resulting resins and cured products of the resins are inferior in characteristics such as heat resistance, water resistance and electrical insulation, and thus they cannot be used in electrical and electronic fields which hate particularly incorporati on of ionic impurities.
Recently, further improvement in heat resistance characteristics, precision and handling workability is demanded for various uses, and as for phenolic resins, increase of molecular weight, uniformalization of molecular structure and high purification have been intensively investigated.
However, when trifunctional phenol compounds are used as the phenol compounds, novolak type phenolic resins obtained by the above processes have a weight-average molecular weight of about 15,000 at the most.
Recently, high-ortho novolak type phenolic resins obtained by reacting a trifunctional phenol compound with an aldehyde compound for a long time in a nonpolar solvent such as xylene have been reported. (For example, JP-A-6-345837). However, average-molecular weight of the resulting resins is about 3,000-40,000, which is still insufficient.
As other high-molecular weight resins obtained using trifunctional phenol compounds, there are known those obtained by adding a phenol compound to greatly excess amounts of hydrochloric acid and formaldehyde (for example, JP-A-57-177011). However, the resulting resins have methylol group and are not novolak type phenolic resins in a strict sense, and, furthermore, they are partially or completely insoluble in solvents, resulting in severe restriction in their use.
As other high-molecular weight novolak type phenolic resins, many examples of resins obtained using, as the phenol compounds, bifunctional phenol compounds such as o-cresol, p-cresol, p-nonyl phenol, and p-tert-butyl phenol are reported. (For example, Narasaki's, "Kogyo Kagaku Zasshi", Vol.66, P391-395 1963, JP-A-50-136393, JP-A-56-92908, JP-A-56-103215, JP-A-57-187311, JP-A-60-260611, and JP-A-61-12714). However, these resins obtained using such bifunctional phenol compounds cannot be completely cured thereafter.
At present, there are no novolak type phenolic resins which can satisfy the demands in various uses, for example, easy handleability in all uses, less ionic impurities and high heat resistance of the cured products in the use of epoxy resin curing agent, excellent curability and high heat resistance and electric characteristics in the use of molding materials, compatibility with rubber and great increase of rubber hardness in the use of rubber compounding.
The object of the present invention is to provide resins which are suitable for various uses such as molding materials for electric and electronic fields, curing agents for epoxy resins, laminates, base resins for carbon materials, and rubber compounding, and cured products of which are excellent in water resistance, heat resistance characteristics and electrical insulation and which can satisfy all of increase of molecular weight, solubility in solvents, high purification, and uniformalization of molecular structure which could not be simultaneously satisfied by the conventional novolak type phenolic resins.